1. Field of the Invention
The present invention relates to a multi-pass printing method in a serial inkjet printing apparatus, and more particularly, to multi-pass printing executed jointly with column-thinning printing in which print scans are conducted while thinning data in groups of columns.
2. Description of the Related Art
A technique referred to as multi-pass printing is often implemented in serial inkjet printing apparatus that print an image by alternately conducting print scans, which move an ink-ejecting print head with respect to a print medium, and conveyance operations, which convey the print medium in a direction that crosses the print scans. With multi-pass printing, an image in the same image region is completed by multiple print scans of the print head while interposing conveyance operations which are shorter than the print width of the print head. For this reason, variation in individual printing elements and variation in conveyance operations do not become concentrated in a single location. Consequently, if multi-pass printing is conducted, the output of a uniform, high-quality image may be anticipated. On the other hand, since such multi-pass printing involves more print scans than usual, there is a problem in that the print time increases significantly.
In contrast, a printing method called column thinning is disclosed in Japanese Patent Laid-Open No. 2002-29097 and Japanese Patent Laid-Open No. 2004-1560, for example. Column thinning is a printing method in which a print head is moved with respect to pixel columns arrayed in the main scan direction of the print head while periodically conducting ejection operations on only a thinned subset of pixel columns. Thus, in the case of column thinning, the ejection operations in each print scan are conducted at a lower pixel density than the pixel density by which pixels are actually arrayed.
If such column thinning is conducted, it is possible to print images of higher resolution than the resolution printable with individual print scans. Also, in the case where the translational velocity of the print head is determined by an upper limit on the ejection frequency of the printing elements, it is possible to increase the translational velocity in the main scan direction while in a state where an upper limit value on the print head ejection frequency has been set. In other words, multi-pass printing with column thinning makes it possible to output images in less time than multi-pass printing without column thinning.
FIGS. 17A and 17B illustrate printing states in 4-pass multi-pass printing with 2-column thinning. For the sake of simplicity, the case of a print head with 16 printing elements will be described herein. Referring to FIG. 17A, a prepared mask pattern 10 is 16 pixels tall, which is equal to the number of printing elements in the print head, and 8 pixels wide. Herein, pixels illustrated as black represent print allowed pixels where the printing of a dot is allowed, whereas pixels illustrated as white represent print non-allowed pixels where the printing of a dot is not allowed. In the case of 4-pass multi-pass printing, the mask pattern 10 is divided into four blocks 1 to 4 which correspond to four printing elements each, and which exist in a complementary relationship with each other regarding the arrangement of print allowed pixels and print non-allowed pixels. The print allowed ratio of each block is 50%.
In the case of 2-column thinning, scans that print on odd columns 11a and scans that print on even columns 11b are repeated in alternation. Thus, in practice, from among the pixels whose printing is allowed by the mask pattern 10, a pixel pattern 12a corresponding to the odd columns 11a is printed by odd scans, and a pixel pattern 12b corresponding to the even columns 101 is printed by even scans.
FIG. 17B illustrates how an image is printed on a print medium by the above 4-pass multi-pass printing. The print scans alternately print in accordance with the pixel patterns 11a and 11b, and conveyance operations equivalent to four pixels (16 pixels/4 passes) are conducted between each print scan. Take the region A of the print medium. In this region, printing is conducted in the following order: odd column scan according to the pixel pattern 11a, even column scan according to the pixel pattern 11b, odd column scan according to the pixel pattern 11a, and even column scan according to the pixel pattern 11b. Meanwhile, in the region B adjacent to the region A, printing is conducted in the following order: even column scan according to the pixel pattern 11b, odd column scan according to the pixel pattern 11a, even column scan according to the pixel pattern 11b, and odd column scan according to the pixel pattern 11a. Although the order of odd scans and even scans differs, the regions A and B are alike in that an image is printed by two odd column scans according to the pixel pattern 11a and two even column scans according to the pixel pattern 11b. Additionally, an image is printed according to the same order as either the region A or the region B in all regions of the print medium.
In this way, in 4-pass multi-pass printing with 2-column thinning, a mask pattern with a print allowed ratio of 50% (100%/2) is prepared such that 2-pass printing can be respectively conducted for odd columns and even columns. If such a mask pattern is prepared, then 4-pass multi-pass printing can be conducted in all regions by respectively conducting 2-pass multi-pass printing on the odd columns and the even columns.
To explain in more general terms, in multi-pass printing with M-column thinning of the related art, a mask pattern with a print allowed ratio of 100%/N is prepared such that N-pass printing can be conducted on individual columns. Then, M×N-pass multi-pass printing can be conducted in all image regions by conducting N-pass multi-pass printing on M groups of columns.
However, with methods of the related art as discussed above, the number of passes on the same image region is restricted to being a value that is a multiple of the column thinning number M. For example, in the case where the column thinning number M=2 as in FIGS. 17A and 17B, the multi-pass number on the same image region must be an even number (2×N), and multi-pass printing with an odd number of passes cannot be realized. Additionally, in the case where the column thinning number M=4, the multi-pass number on the same image region must be a multiple of 4, and printing with other numbers of passes cannot be conducted.
Setting the multi-pass number in an inkjet printing apparatus to a suitable value is important for maintaining a balance between image quality and printing speed, and the existence of restrictions of the multi-pass number as discussed above is undesirable.